feat: add generated documentation for OpanAPI clients

Generate documentation for each of the OpenAPI clients along with
redoc-based documentation for the schema itself. Downstream projects
should arrange to copy this documentation to some appropriate place.

We add generic "document a TypeScript project" and "document a Python
project" jobs along with specialisations for each of the clients we
generate.

All documentation ends up in `OPENAPI_GENERATOR_DOCS_ARTIFACT_DIR` which
can be overridden by consuming projects.